Objective  To address the lack of understanding of the potential and limitations of imbibition and oil displacement in tight reservoirs, imbibition and oil displacement experiments were conducted using tight sandstone cores to clarify its effectiveness and mechanisms.

Method An online nuclear magnetic resonance (NMR) imbibition experimental setup for tight sandstone cores was established. Counter-current imbibition experiments were conducted using different imbibition agents, including simulated formation water and surfactant solutions, to evaluate the enhancing effect of surfactant on imbibition.

Result The imbibition and oil displacement process in tight oil reservoirs can be divided into two stages: an initial high-speed oil displacement stage and a late slow-speed oil displacement stage. Compared with single surfactants, the composite surfactant system exhibits better oil displacement performance, and its efficiency is positively correlated with reservoir properties. The maximum imbibition and oil displacement efficiencies of the composite surfactant solution in lithic fine sandstone and dolomitic lithic fine sandstone are approximately 12.49 and 5.41 percentage points higher than those of the single surfactants respectively. Compared with simulated formation water, the addition of composite surfactants significantly increases the initial high-speed oil displacement rate by 7.15 percentage points per hour, the duration of the initial high-speed stage by 2.0 d, and the ultimate imbibition distance by 1.95 cm. In lithic fine sandstone, the proportions of micropores and mesopores are large, contributing significantly to the overall imbibition and oil displacement efficiency. The addition of composite surfactants significantly enhances the mobilization of crude oil in various pores of tight sandstone reservoirs, with a larger increase in micropores and mesopores, averaging 17.76 percentage points.

Conclusion The study shows that, the imbibition and oil displacement distance in tight oil reservoirs is on a centimeter scale, and composite surfactants can significantly improve the imbibition and oil displacement efficiency.